1. Field of the Invention
This invention relates to camera systems and in particular to a camera system for optically scanning moving objects to obtain optically encoded information from the surface of the objects.
2. Background Art
Merchandise, various component parts, letters, moving objects, containers and a whole gamut of related items being shipped or transported, frequently must be identified with information regarding origin, flight number, destination, name, price, part number and numerous other kinds of information. In other applications, the reading of encoded information printed on labels affixed to such items permits automation of sales figures and inventory as well as the operation of electronic cash registers. Other applications for such encoded labels include the automated routing and sorting of mail, parcels, baggage, and the like, and the placing of labels bearing manufacturing instructions on raw materials or component parts in a manufacturing process. Labels for these types of articles are conventionally marked with bar codes, one of which is the Universal Product Code. Numerous other bar code systems are also known in the art.
However, certain applications require the encoding of larger amounts of information on labels of increasingly smaller size. Commercially-available bar code systems sometimes lack sufficient data density to accommodate these needs. Attempts to reduce the overall size and spacing of bars in various bar code systems in order to increase data density have not solved the problem. Optical scanners having sufficient resolution to detect bar codes comprising contrasting bars spaced five mils or less apart are generally not economically feasible to manufacture because of the close tolerances inherent in the label printing process and the sophisticated optical apparatus required to resolve bit-encoded bars of these dimensions. Alternatively, to accommodate increased amounts of data, very large bar code labels have been fabricated, with the result that such labels are not compact enough to fit on small articles. Another important factor is the cost of the label medium, such as paper. A small label has smaller paper costs than a large label. This cost is an important factor in large volume operations.
Therefore, other types of codes have been investigated to overcome the problems associated with bar codes. Some alternatives to bar codes are: circular formats using radially disposed wedged-shaped coded elements, such as those disclosed in U.S. Pat. No. 3,553,438, issued to Blitz, and entitled "Mark Sensing System", or concentric black and white bit-encoded rings, such as in U.S. Pat. Nos. 3,971,917 and 3,916,160, issued to Maddox and Russo, respectively; grids of rows and columns of data-encoded squares or rectangles, such as in U.S. Pat. No. 4,286,146, entitled "Coded Label and Code Reader for the Coded Label," issued to Uno; microscopic spots disposed in cells forming a regularly spaced grid, as disclosed in U.S. Pat. No. 4,634,850, entitled "Quad Density Optical Data System", issued to Pierce; and densely packed Multicolored data fields of dots or elements, such as those described in U.S. Pat. No. 4,488,679, entitled "Code and Reading System, " issued to Bockholt.
These codes were satisfactory for many applications. However, some of the encoding systems described in the foregoing examples and other encoding systems known in the art still did not provide the required data density. For example the encoded circular patterns and grids of rectangular or square boxes did not provide sufficient density. Alternatively, in the case of the grids comprised of microscopic spots or multi-colored elements referred to above, such systems require special orientation and transport means, thus limiting the utility to highly controlled reading environments. A further improvement, U.S. Pat. No. 4,874,936, entitled "Hexagonal Information Encoding Article, Process and System," issued to Chandler discloses a label for storing information-encoded hexagons which stores densely packed information and may be read at high speed in any direction. This improvement thus solves the data density problems associated with bar codes.
However, the newer encoding systems, including the encoding system taught by Chandler, are of formats which are entirely different from conventional bar codes and can not be read by conventional bar code readers. Therefore it is difficult to use the newer encoding methods which may solve the data density problems of bar codes in an environment in which bar codes are also present unless separate scanning and decoding equipment is provided for each type of code. Thus, it would be advantageous to have a single scanning and decoding device which may detect and decode different types of encoding systems when the different encoding systems are alternately disposed in the range of the optical scanning and decoding device. Additionally, when higher density codes are used higher resolution optical scanning and therefore higher levels of illumination are required. However, the very high levels of illumination are only required some of the time. Thus wasted is energy and a threat of eye injury is needlessly created during the remaining periods.
Regardless of the type of encoding system used, high quality detection is required in many applications. Modern conveyor systems may have conveyor belt widths of three to four feet over which the position of an information-encoded label may be disposed and belt speeds of five hundred feet per minute or more. They carry moving objects which may be of varying heights upon which information-encoded labels are disposed. Thus, it can be very difficult for optical decoding systems to locate and read the data encoded labels disposed on these rapidly moving objects.
These problems have led to the need for providing a simple, rapid and low-cost means of signaling the presence of a data-encoded label within the field of view of an optical scanner mounted in a manner to permit scanning the entire conveyor belt. It is known in the art to solve these problems by providing easily recognizable optical acquisition targets as part of an encoding system. For example, the system taught by Chandler uses a concentric ring acquisition target for this purpose.
Bar code systems may also be understood to provide an acquisition target. For example, it is conventional in the art of detecting bar codes to pre-detect the rectangular shape formed by the bars. In this type of system a rectangle may indicate the presence of a bar code. Conventional bar code detectors, after acquiring the rectangle, then attempt to find encoded data within the pre-detected rectangle. If valid data is found encoded within the rectangle, the bar code is thus detected. However, many other types of rectangles within the range of the optical scanning device may cause false pre-detects in this method.
Further data arrays having acquisition targets other than the concentric rings and bar codes are known in the art. For example, concentric geometric figures other than rings, such as squares, triangles, hexagons and numerous variations thereof, are described in U.S. Pat. No. 3,513,320, issued to Weldon, on May 19, 1970, and entitled "Article Identification System Detecting Plurality of Colors Disposed on an Article", and U.S. Pat. No. 3,603,728, issued to Arimura, on Sept. 7, 1979, and entitled "Position and Direction Detecting System Using Patterns". U.S. Pat. No. 3,693,154, issued to Kubo et al., on Sept. 19, 1972, and entitled "Method For Detecting the Position and Direction of a Fine Object", and U.S. Pat. No. 3,801,775, issued to Acker, on Apr. 2, 1974, and entitled "Method and Apparatus for Identifying Objects" also describe systems using symbols comprising concentric circles as identification and position indicators, which symbols are affixed to articles to be optically scanned.
U.S. Pat. No. 3,553,438, entitled "Mark Sensing System", issued to Melvin, discloses a circular data array having a centrally-located acquisition target comprising a number of concentric circles. The acquisition target of Melvin provides an image which may be used by an optical scanning device to locate the label. The acquisition target of Melvin also permits determination of the geometric center of the label and the geometric center of the data array. This is done through logic circuitry which recognizes the pulse pattern representative of the concentric ring configuration.
The foregoing systems are generally scanned with an optical sensor capable of generating a video signal output. The video output signal corresponds to the change in intensity of light reflected off the data array and is therefore representative of the position and orientation of the scanned symbols. The video output of such systems, after it is digitized, has a particular bit pattern which may be matched to a predetermined bit pattern. A common bit pattern of this type is a simple harmonic as in the system taught by Chandler.
It is well known to detect the presence of harmonics such as those produced by these systems in both the digital and the analog domains. However, in high speed optical systems for acquiring digital data the recognition of the target must take place in much less time than is available to recognize, for example, the touch tone of a telephone. Thus, a system for detecting any of these codes must reliably identify the harmonics caused by an optical scan of a common optical acquisition target from a signal which lasts only as long is the acquisition target is actually scanned.
As previously described, Chandler discloses a circular data array having a centrally located acquisition target comprising a series of concentric rings which produces a harmonic scan output signal. The acquisition target of Chandler provides a means of acquiring the circular label by the optical sensor and determining its geometric center and thereby the geometric center of the surrounding data array. This is done through logic circuitry which operates to recognize the pulse pattern representative of the concentric ring configuration of the acquisition target.
This recognition method relies upon a one dimensional scan of the concentric ring pattern. When the concentric ring acquisition target is advanced by a conveyor belt to the scan line of the optical scanning equipment, the scan line eventually passes through the center of the concentric rings. At that point, the harmonic scan output signal is provided at the output of the optical scanning device. This harmonic scan signal is then detected by a correlation filter. Alternately it may be detected by any other type of harmonic detection device. However, this system is subject to some false detects since other objects scanned by the optical scanning device may also provide an harmonic signal at substantially the same frequency as the concentric ring acquisition target. Another system teaching concentric ring detector of this nature is taught by Shaw in U.S. patent application Ser. No. 07/728,219, filed Jul. 11, 1991.
The system set forth in Chandler solves many of the problems of the prior art systems by providing very high data density as well as a reliable system for target acquisition. However, in addition to the problem of false detects due to the one-dimensional scan, a relatively high resolution scanning of this label is required in order to acquire the target as well as to decode the high density data. An optical scanning system capable of scanning the higher density data of the codes which solve the density problems of bar codes may therefore be more complex and costly than a system which is adapted to merely acquire a low resolution target.
Thus it is often necessary for optical scanning systems to acquire a target under very difficult circumstances. The target acquired may appear at different locations within the scanning field and may be moving rapidly. In addition to these problems the acquisition target may be disposed at varying distances from the optical scanning device. For example, labels on moving objects may be scanned at varying distances from the scanning device because of varying package sizes. This introduces magnification into the sampled sequence acquisition target. The closer the acquisition target is to the scanning device, the larger it appears and the lower the frequency of the sampled sequence. Larger scanning distances produce higher frequencies. Detection of the varying frequencies caused by varying amounts of magnification can be difficult since digital filters with adjustable poles and zeros may be expensive and complicated. Additionally the varying distance introduces the need for focussing in order to accurately scan the acquisition target.
There are two common solutions to these problems known in the prior art. One common solution to the focusing problem known in the prior art is using a depth of focus sufficient to permit detection of acquisition targets at varying distances from the optical scanning device. Another common solution to the magnification problem is fixing the distance between the optical scanning device and the acquisition target in order to prevent magnification.
Prior art references teaching the use of a large depth of focus in order to avoid focusing problems include: U.S. Pat. No. 4,544,064, entitled "Distribution Installation for Moving Piece Goods", issued to Felder; U.S. Pat. No. 3,801,775, entitled "Method and Apparatus for Identifying Objects", issued to Acker; U.S. Pat. No. 3,550,770, entitled "Method for Automatic Sorting or Recording of Objects and Apparatus for Carrying Out the Method", issued to Lund, and U.S. Pat. No. 4,454,610, entitled "Methods and Apparatus for the Automatic Classification of Patterns," issued to Sziklai.
One example of a reference teaching a fixed distance between the acquisition target and the optical scanning device include: U.S. Pat. No. 3,971,917, entitled "Labels and Label Readers", issued to Maddox et al Another reference teaching this is U.S. Pat. No. 3,757,090, entitled "Mechanical Reading and Recognition of Information Displayed on Information Carriers", issued to Haefeli, et al.
A solution to both the focusing problem and the magnification problem is adjusting the distance between the acquisition target and the optical scanning device. U.S. Pat. No. 4,776,464, issued to Miller, teaches this type of adjustment. However, this method is mechanically difficult for a large number of quickly moving and closely spaced moving objects of widely varying heights. Additionally, the system taught by Shaw taught in U.S. patent application Ser. No. 07/728,219 teaches a similar solution to this problem.